This disclosure is directed to a wireless communications network overlay for determining the location of mobile appliances.
The use of wireless communication devices such as telephones, pagers, personal digital assistants, laptop computers, etc., hereinafter referred to collectively as “mobile appliances”, has become prevalent in today's society. Recently, at the urging of public safety groups and others, there has been increased interest in technology which can determine the geographic position, or “geo-locate” a mobile appliance in certain circumstances. For example, the Federal Communication Commission (FCC) has issued a geo-location mandate for providers of wireless telephone communication services that puts in place a schedule and an accuracy standard under which the providers of wireless communications must implement geo-location technology for wireless telephones when used to make a 911 emergency telephone call (FCC 94-102 E911).
In addition to E911 emergency related issues, wireless telecommunications providers are developing location-enabled services for their subscribers including roadside assistance, turn-by-turn driving directions, concierge services, location-specific billing rates and location-specific advertising.
In a network-based geo-location system, the mobile appliance to be located is typically identified and radio channel assignments determined by (a) monitoring the control information transmitted on a radio channel or a wireline interface within the wireless network for telephone calls being placed by a mobile appliance to thereby detect calls of interest, e.g., 911 calls, or (b) a location request provided by a non-mobile appliance source, e.g., an enhanced services provider. Once a mobile appliance to be located has been identified and radio channel assignments determined, the geo-location determining system is first tasked to determine the geo-location of the mobile appliance and then directed to report the determined position to the requesting entity or enhanced services provider.
The monitoring of the RF transmissions from the mobile appliance or from a wireline interface to identify calls of interest is known as “tipping” and generally involves recognizing a call of interest being made from a mobile appliance and collecting the call setup information. Once the mobile appliance is identified and the call setup information is collected, the location determining system can be tasked to geo-locate the mobile appliance.
FIG. 1 shows a conventional mobile-appliance communication system having a mobile switch controller 45 connected to base stations 10 for communicating with a mobile appliance 20. Each base station 10 contains signal processing equipment and an antenna for transmitting to and receiving signals from the mobile appliance as well as other base stations and centrally located control and processing stations. A mobile appliance location determining sensor 30, sometimes referred to as a wireless location sensor (“WLS”), may be positioned at some or all of the base stations 10 to determine the location of a mobile appliance within the signal coverage area of the communication system. The antenna may be a multi-element antenna. The Mobile Positioning Center 40 (MPC) generally tasks the location system to locate a mobile appliance, and supplies channel assignment information to the location system via the GCS 50.
A network overlay system is generally composed of two main components, one component which resides at the base station that makes measurements on an RF signal emanating from a wireless device, known in the art as a geo-location sensor or a wireless location sensor (“WLS”) 30, and the other component which resides at a mobile switch that tasks the geo-location sensor groups to collect data and then uses the data to compute a location estimate. This component is generally referred to as the Geo-location Control System 50 (“GCS”).
As discussed above, one way to initiate a geo-location evolution is for an outside entity to task the GCS to generate a location estimate on a particular (target) mobile appliance. The tasking may be accompanied by information on the mobile of interest including the serving base station and sector for the call as well as the RF channel being used by the wireless communications network to complete the wireless connection. Typical examples of information that must be known include the RF frequency especially where Frequency Division Multiple Access (“FDMA”) is used, the time slot of the transmission for Time Division Multiple Access (“TDMA”) systems, and the spreading code for Code Division Multiple Access (“CDMA”) systems. In general, this information is used to tune the radio receivers of the WLSs so that measurements related to the geo-location estimation can be made. Once the GCS receives the geo-location tasking, the GCS tasks a set of WLS units to take measurements of the RF emission of the target mobile. The WLS units take the measurements and report the data from the measurements to the GCS. The GCS then computes a location estimate using a mathematical or data matching algorithm, as is well known in the art.
Alternatively, a geo-location evolution may commence upon detection of a call of interest, such as a 911 call. In this scenario, control channels used to set up calls in the wireless network can be scanned to detect the placement of a call of interest. The signaling that occurs on the control channel can be used to determine location if the control data is contained on an RF channel, or alternatively the RF traffic channel parameters can be extracted from the control channel messaging to determine which traffic channel to use for geo-location related measurements.
In operation, these network overlay location systems typically take measurements on RF transmissions from mobile appliances at base station locations surrounding the mobile appliance and estimate the location of the mobile appliance with respect to the base stations. Because the geographic location of the base stations is known, the determination of the location of the mobile appliance with respect to the base station permits the geographic location of the mobile appliance to be determined. The RF measurements of the transmitted signal at the base stations may include the time of arrival (“TOA”), time difference of arrival (“TDOA”), the angle of arrival (“AOA”), the frequency difference of arrival (“FDOA”), the signal power, or the unique/repeatable radio propagation path (radio fingerprinting) derivable features. In addition, the geo-location systems can also use collateral information, e.g., information other than that derived for the RF measurement to assist in the geo-location of the mobile appliance. Such collateral information may include the location of roads, dead-reckoning, topography, map matching etc.
Network overlay location systems typically locate a mobile appliance on the traffic channels of a wireless network. Network overlay systems typically use sensors employing techniques of TDOA supplemented with AOA to perform a multi-site location computation. The traffic channel assignment information is provided through a separate process, with one option being a wire line interface 41 (see FIG. 1) providing MOBINFO (IS-41 mobile information) parameters passed by the Mobile Positioning Center (“MPC”) 40 as part of the GPOSREQ (J-STD-036 Geolocation Position Request) message.
In general, the overlay system tunes radio receivers of the WLS to an RF channel occupied by the mobile appliance of interest to make these measurements. As noted above, information about the channel must be known for the radio receivers in the sensors to be tuned. One class of air interfaces that cannot be located with the previously defined information used in the prior art are frequency hopping interfaces. Frequency hopping signals change RF frequency rapidly over time. For example a frequency sequence {right arrow over (f)} is comprised of a sequence of N different frequencies {right arrow over (f)}=(f0, f1, f2, f3 . . . fN-1) with each hop typically having a duration T. Frequency hopping, as is well known in the art, is used in mobile radio applications to mitigate the effects of stationary interference or multi-path that may exist if the transmission is on a certain RF frequency. By rapidly changing frequencies, the probability of prolonged interference or fading due to multi-path is reduced.
In general the hopping sequences and duration of each hop are deterministic and known by both the mobile appliance and the serving base station. However, even if the hopping sequence is known, the radio receivers in the geo-location system cannot know where in the sequence to look for the mobile appliance signal, and location estimates determined from measurements made without this information can be made on the wrong mobile appliance since a signal from other than the target mobile appliance may hop into the RF channel being measured.
In communications between the base station and mobile appliance, the mobile's frequency hopping transmission on the reverse channel will be synchronized in some manner to the forward channel transmission as is known in the prior art. This synchronization is achieved for some air interfaces by having the mobile appliance use the reception and timing of broadcast control frames from the base station to establish a timing grid for hopping its transmission RF frequency. By having the timing grid, and being told what hopping pattern to use including the first RF frequency in the pattern, the base station and mobile appliance can know what frequency each will transmit on in time. Further refinement of the hopping timing is done by having the base station advance transmission in time to compensate for range uncertainties from the base station. This is accomplished by having the base station send timing advance (TA) control data to the mobile on forward transmission channels while during the course of traffic transmission. This synchronization between the forward (typically taken to be from the base station to the mobile appliance) and reverse (typically taken to be from the mobile appliance to the base station) channels is required for mobile appliance and base station communications. The forward channel transmission may or may not be synchronized to any other reference at the serving base station or another base station. When it is synchronized, sources of synchronization include GPS time, and timing derived from communications facilities used by the base station such as T1 or E1 timing.
The subject matter of the present disclosure obviates the deficiencies in the prior art by providing a system and method for the radio receivers of the wireless location sensors to be tuned to follow the hopping sequence to facilitate measurement of a signal from a target mobile appliance that is using a frequency hopping air interface.
It is an object of the disclosure to present a novel system and method for geo-locating a frequency hopping mobile appliance. The novel system and method may monitor communication between a base station and a target mobile appliance on a forward channel, retrieve synchronization information from the forward channel; and synchronize the wireless location sensors with the mobile appliance on the reverse channel. The measurements of the location system may be made by a sensor located at one base station or other site on the forward and reverse transmissions from wireless communications system equipment co-located at that site, or from wireless communications equipment transmissions located at neighbor sites. Thus timing synchronization and other information necessary to follow frequency hopping patterns on forward and reverse transmissions can be accomplished without having location system sensors at each wireless communications system base station site. The system and method may also include measuring an attribute of the mobile appliance's signal to thereby generate an estimate of the geo-location of the mobile appliance based at least in part upon the measured attribute.
In is also an object of the disclosure to present a novel improved method for geo-location of a wireless appliance including the steps of retrieving channel assignment information from an MPC or by monitoring an RF or wireline link for channel assignment information, monitoring the reverse channel at a plurality of sensors for the wireless appliance's signal, measuring an attribute of the signal at the plurality of sensors, and determining the location of the wireless appliance from the measured attributes. The improved method may also include monitoring a forward link of the mobile appliance and retrieving synchronization information from the forward link to thus enable measuring an attribute of the signal and for geo-locating the mobile appliance.
It is still another object of the disclosure to present, in a wireless communication system communicating with a frequency hopping mobile appliance with a determinable hopping frequency and duration, a novel network overlay geo-location system for geo-locating the mobile appliance. The geo-location system may include a plurality of sensors located at the plural base stations for measuring an attribute of a reverse communication channel, a geo-location system controller operably connected to each of the plurality of sensors, and an information channel for transmitting information regarding the mobile appliance's frequency hopping characteristics to the plural wireless location sensors. In the system, the forward channel may contain synchronization information for tuning the plurality of sensors to the reverse communication channel, thus allowing the sensors to collect measurements to geo-locate the mobile appliance.
It is another object of the disclosure to present a novel method for geo-location of the wireless appliance including the steps of monitoring the forward channel for synchronization information and using the synchronization information to tune the plurality of sensors to the reverse channel.
These and other advantages of the disclosed subject matter over the prior art will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments.